Unban IP blocked by fail2ban

Login to the Server as root Use to and search for the banned ip. If you can't login: Try using a different IP. You may use you Mobile phone thetering. 

iptables -L -n

You'll get something like

Chain fail2ban-pam-generic (1 references)
target     prot opt source               destination
RETURN     all  --  0.0.0.0/0            0.0.0.0/0
Chain fail2ban-ssh (1 references)
target     prot opt source               destination
DROP       all  --  193.xxx.2.142        0.0.0.0/0
DROP       all  --  202.120.xxx.87       0.0.0.0/0
DROP       all  --  88.xxx.223.148       0.0.0.0/0
RETURN     all  --  0.0.0.0/0            0.0.0.0/0

Check which of the rules (e.g. fail2ban-pam-generic or fail2ban-ssh) the banned ip is targeted. Now you can remove the ip from iptables:

iptables -D fail2ban-pam-generic -s [BANNED IP] -j DROP